(1) One of the cDNA clones obtained from Xenopus anterior pituitary RNA by hybridization to mouse POMC cDNA was mapped with restriction nucleases and partially sequenced by chemical methods. One portion of the molecule has no homology to pro-opiomelanocortin mRNAs (POMC) of other animals, but contains a 134 bp region with 80% homology to a repetitive genomic sequence called REM1. There is also, within this same region, a stretch of 12 bp which is 100% homologous to a region in the 5' untranscribed region of the human POMC gene. The other half of the insert has been partially sequenced. An open reading frame contains a region of some amino acid homology to bovine POMC, but an expected area of high homology has proved difficult to sequence. (2) In the neuroblastoma project, libraries of cDNA presumed to be differentiation-specific obtained by cascade hybridizations, have been screened by hybridizations to cDNA probes made from undifferentiated and differentiated cell mRNA. Several colonies which appear to be differentiation-specific or enriched have been selected and are being characterized. (3) Sequences in one library each of cascade-purified cDNAs of undifferentiated and differentiated S20Y cells are being systematically examined as to abundance of the mRNA, specificity for the differentiation stage from which they came and for the presence or absence of restriction fragment polymorphisms in digests of DNA from cells in the two differentiation states as well as from mouse brain and liver. (4) Screening of the Lamdagt11 cDNA library prepared last year from differentiated NG108-15 cell mRNAs with monoclonal antibodies directed against rat retina revealed a clone, LamdaGDT7, with a 300 bp insert which corresponds to an antigen with a restricted distribution in the rat nervous system. This clone has been subcloned into M13 mp8/9 for sequencing. (5) A cDNA clone, Lamda ChAT7 was isolated by use of a polyclonal antibody to human choline acetyltransferase (ChAT) from a Lamdagt11 cDNA library (a gift from Dr. R. Lazzarini) made from mRNAs from human basal ganglia. Several lines of evidence indicate that this 1100 bp insert is complementary to ChAT mRNA.